The first known member of "The Mile High Club" was Lawrence Sperry in 1916

A website using the name Mile High Club regards the "Club's" "founder" as pilot and design engineer Lawrence Sperry[5] along with "socialite Mrs. Waldo Polk" citing their possibly apocryphal flight in an autopilot-equipped Curtiss Flying Boat near New York in November 1916.[6][7][8]@Curionic